(1) Field of the Invention
The present invention relates generally to a control circuit and method for controlling the luminous intensity of a discharge lamp such as a utilized as head lamp in automotive vehicles.
(2) Background of the Art
A Japanese Patent Application First Publication No. Showa 62-198046 published on Sep. 1, 1987 (which corresponds to U.S. patent application Ser. No. 945,679 and to a British Patent No. 2186957 issued on Feb. 21, 1990) exemplifies a combination vehicle light.
In the previously proposed Japanese Patent Application First Publication, the automotive head lamp includes a pair of dischargeable electrodes and a discharge lamp having a substance which vaporizes and emits the light during its discharge. The, the discharge lamp is installed within a lamp housing. An optical lens is installed on a front face of the lamp housing and a reflector is installed on a rear inner surface thereof, respectively. The discharge lamp is charged when the metallic vapor of the light emitting substance is excited at a high energy state, and, when it is again returned into a low energy state the lamp is caused to illuminate.
However, in order to stabilize the luminous intensity an automotive vehicle head lamp, it is necessary to sufficiently vaporize the molecules of a luminous metal. The degree of vaporization of the luminous metal is dependent on the temperature within the discharge lamp. Therefore, if a light switch is operated when a cold state exists within the discharge lamp, it takes time to reach a prescribed luminous intensity.
Hence, in transient cases where the head lamp is illuminated while the vehicle enters a tunnel in day time, or the head lamp is illuminated after being turned off for a while during night time, or beam switching illumination is carried out and the head lamp is illuminated at alternating intensities, the prescribed intensity value cannot speedily be obtained at the moment a lighting operation is required.
To get around this problem head lamp structures have been proposed in which an incandescent or halogen lamp is used together with the discharge lamp and until the discharge lamp arrives at the prescribed luminous intensity value, the incandescent lamp or halogen lamp is illuminated as an auxiliary lighting means.
However, in such a system, since both the discharge lamp and the incandescent or halogen lamp cannot be installed within the same lamp housing it is necessary to contrive a special structure for supporting two types of lamps. Therefore, the construction of the head lamp becomes complex and expensive.